The aim of this research is to elucidate the control of the differentiation of a multipotent stem cell, the interstitial cell (i-cell) of hydra. The approach is to alter the cell composition of intact animals and study the i-cell division and differentiation behavior. The cell composition can be altered by the selective deletion of cell types in intact animals and with a cell aggregation technique. This involves dissociating hydra into single cells and forming cell aggregates which develop into normal animals. Separation of cell types will permit construction of aggregates (hence animals) of selected cell composition. The pattern of i-cell behavior will be analyzed quantitatively as methods have been developed for quantitatively measuring all cell populations in hydra, as well as examining quantitatively the behavior of a cohort of isotopically-labelled cells with autoradiography. The objects of the are to: (1) study the basis of position-dependent differentiation of interstitial cells; (2) study the role of intersttial cell migration in position-dependent differentiation, (3) study the complexity of the interstitial cell system; (4) develop a method for injecting single interstitial cells into intact hydra in order to study the behavior of individual interstitial cells into hydra in order to study the behavior of individual interstitial cells in living hosts.